Interface between optical fiber and device

For optical communication, optical interfaces of devices are connected through optical fiber. For example, the connection between OLT and ONU (generally speaking, SFP optical module is required to provide optical interface connection on OLT), and the data transmission between two optical fiber transceivers is also conducted through optical fiber, so the name of their interface becomes the most basic knowledge in optical communication.

There are two types of optical fiber interfaces: by structure and by end face. These two classifications are not mutually exclusive, but coexist. Like SC/APC in the figure below, the former SC is one of the fixed classification, and the latter APC is one of the end classification.

Structure classification

1. FC round thread

The FC interface of the optical fiber has a slightly raised position, and the FC interface of the opposite device has a gap position. The two need to be aligned. After alignment, insert the optical fiber and rotate the outer structure (nut) to complete the fixation. If the bulge is not aligned with the gap, tighten the nut, and light transmission between the two will have a great loss.
Advantages: After the position is aligned and tightened, the optical fiber and the device are connected firmly.
Disadvantages: The plugging is complex and the installation takes a long time.

2. ST bayonet round type
The ST head is fixed by a bayonet after it is inserted and rotated half a circle. The disadvantage is that it is easy to break.

3. SC bayonet square large mouth
It mainly refers to the fixed position of plug and latch type (the lower left figure is SFP optical module)
Advantages: convenient direct plugging and easy operation
Disadvantages: Compared with FC interface, the connection is not too firm.

4. LC small square mouth
LC is smaller than SC in terms of fiber core and appearance. LC is a modular jack (RJ) latch mechanism

End face classification

1. PC microsphere surface grinding and polishing

PC (Physical Contact) means physical contact. Light comes out from the end of the optical fiber. Although the end face of the end is transparent and allows light to pass through, there will still be some light reflected back, which is called back reflection. The reflected light does not meet the expected transmission direction and needs to be suppressed. The degree of inhibition is called return loss. PC has been gradually replaced by UPC.

2、UPC

UPC (Ultra Physical Contact). UPC is improved on the basis of PC. The end face of UPC is slightly bent. After polishing for a long time, it has a better surface finish and better return loss than PC structure, but it is not solid enough. Repeated plugging and plugging will lead to a decline in surface quality and final performance. The appearance of the optical fiber shell on the UPC end face is generally blue.

3. APC is at an angle of 8 degrees and the microsphere surface is grinded and polished

Compared with PC, APC will have a certain angle inclination, and the end radius of the insert is polished at an angle of 8 °, thus minimizing the back reflection. The optical return loss of APC connector is – 60dB or higher, which is superior to other types of connectors. APC connectors can only be matched with other angle-polished connectors, but not with non-angle-polished connectors, otherwise it will cause high insertion loss. The appearance of the optical fiber shell on the APC end face is generally green.

Other introductions

Flange plate

Connect two optical fiber interfaces to extend the optical path. Note that APC end face and UPC end face cannot be mixed.

Dust cap

It is used for dust prevention to prevent the fiber core end face from being polluted, which will affect the optical signal transmission